In reply to IWhyte:
I would second Glen Croe, given your location. Remember Middle Crag being easy to access and a decent spread of grades. And a lovely south facing mountain environment.
Loudoun Hill and Ben A'an maybe not the best for a beginner group, given the steep terrain.
Auchinstarry and Neilston understandably popular for such things but a fair drive.
Sport crags - fair enough but logistically more complicated if you have to climb to both rig and retrieve your top rope kit (please don't top rope through the lower offs!). However there is a wee sport crag at Ardvorlich that I remember being able to rig from above. Not masses of parking but if you shared a vehicle might work